Monthly Q&A April 2024

Matt Biggs says: This could be due to cold, or a deficiency of nitrogen or potash in the soil. Lupins need full sun and moist but well-drained soil, and dislike waterlogging or cold conditions. If planting them out in lighter or heavier soils where water either disappears quickly or drains away very slowly, first improve the soil with well-rotted organic matter. For plants already in the ground, as long as they are small, you should be able to lift them, improve the soil, and then replant. Water them well, keeping them shaded from strong sunlight if necessary until they re-establish. Feed them with high potash slow-release fertiliser until July and protect from slugs, especially in spring. Keep them well watered, especially when they are newly planted, using rainwater whenever possible. If there's plenty of organic matter in your soil you will not have to water as often, as this helps to hold water around the roots.

I have a damp, mossy lawn with a white flaky mould on it. How can I get rid of this? Julie, Somerset

Ashley Edwards says: This sounds like snow mould or powdery mildew. Both are caused by poor drainage and shade, conditions that moss also loves. It won’t kill the lawn unless it’s a severe case. To help reduce it you can scarify the lawn, add sharp sand for extra drainage and avoid using nitrogen-rich fertilisers toward the end of the year, as these will encourage foliage growth but not strong roots, just when the plants need to be bulking up their roots for winter. You can also prune any overhanging shrubs or trees to increase airflow over the lawn and reduce shade, so water evaporates from the surface more easily. Raking away fallen leaves in the autumn also helps. Moss often performs much better than lawn in a shady, damp garden and is coveted in traditional Japanese moss gardens. By taking the steps above, you should see a reduction in the mould, but I wouldn’t expend vast amounts of energy trying to rid the garden of it.

My camellias are healthy plants but they never flower. What could be the problem with them please? Mairi, North Yorkshire

Matt Biggs says: Camellias should be in a sheltered, partly shaded spot in acidic soil or compost; avoid east-facing locations where early morning sun damages frosted flowers. Flowers are formed within fat buds during summer to bloom the following spring, so it is important that the compost or surrounding soil remains constantly moist from July to October. Feed with ericaceous fertiliser then lay a 7.5cm mulch of pine needles or partially rotted forest bark around the base to conserve moisture. Do this in spring, as feeding after July may cause bud drop. Water with rainwater; tap water can be used for short periods during periods of drought.

The mature bramley apple tree on my allotment is affected by bitter pit, making any stored apples inedible. The tree is too big to spray so what other treatments can I use please? Karen, West Sussex

Ashley Edwards says: This is characterised by brown sunken pits on the apples, which causes them to rot in storage. The good news is that this can normally be corrected by changes to nutrient and water levels. The first step is to make sure the tree receives plenty of water through summer. As it is a mature tree the best way to do this is to add plenty of organic matter in the form of an autumn or spring mulch. This will increase the soil's water-holding capacity if repeated annually. Thinning out the tree can also help to reduce the tree’s need for water. With hot, dry spells becoming more common in summer, this is likely to affect more apple trees across the country.

• 8 things you need to know about growing apples

My neighbour gives me a compost bag of used hay and droppings from her rabbits' hutch every week. I have been putting it straight on the veg patch and beds as a mulch, but would it be better if I added it to the compost heap for a few months? Emma, North London

Matt Biggs says: The hay and rabbit droppings would be better added to the compost heap and allowed to rot down, then used as a soil conditioner. Compost heaps should contain a balanced mix of slow- and fast-rotting material. Slow-rotting material is things such as woody stems, straw and cardboard; while fast-rotting ingredients are things like leaves, young weed seedlings and green material. Hay rots more slowly than fresh grass as it has been dried, so spread it over the surface of the heap in shallow layers or mix it in with other fresh material, then water. Compost heaps should be moist but not waterlogged. Cover your heap with an old carpet or similar.

How can I improve my shrub rose, growing in a north-east-facing bed? Its stems are so thin that they cannot support their flowers, and the stems just flop. Brian, Staffordshire

Ashley Edwards says: There are many roses that will grow on shadier walls – 'Claire Austin', 'The Generous Gardener' and 'Rambling Rector' are just a few. Without knowing which variety you have; it could be that your rose is a sun-lover, in which case a south-facing aspect would perhaps be better suited. It is also important to prune shrub roses hard in the winter down to a goblet structure to allow maximum airflow and strong stems. After pruning make sure to mulch with a good organic fertiliser – well-rotted garden compost works well.

How do I stop the three-cornered leeks at the top of my garden trying to colonise the rest of my garden? I get in among them at this time of year, removing flowerheads and leaves, but otherwise leave them. Sharon, Hampshire

Matt Biggs says: The three-cornered leek (or three-cornered garlic), Allium triquetrum, is named as such because of the shape of its leaves and odour. It spreads rapidly by bulbils and seeds, which are dispersed by ants, and is listed as an invasive species in the UK Wildlife & Countryside Act, so it should not be introduced in wild situations. It can be grown in gardens but should be kept under control. Dig out plants in spring when the leaves first appear and compost separately in sealed bin liners, or take them to a licensed landfill site. Alternatively, hoe off the leaves as they emerge, repeating the process until the plants are weakened and die. It may take several years to exhaust the plant in the area you are working.

My herb seedlings don't survive. They all germinate well in my warm kitchen. When they're about 1-2cm the stalks shrivel up then the seedlings die. What am I doing wrong? Louise, East Yorkshire

Ashley Edwards says: This sounds like damping-off, a common fungal problem with small seedlings, which is more likely to occur when the soil is very wet. Avoid overwatering, and consider watering from below so no water hits the foliage of the seedlings. You can do this by sitting the tray in some shallow water for a few minutes and allowing it to seep in. Take the tray out of the water as soon as the surface looks moist. Also make sure your seed trays or pots have plenty of drainage holes at the bottom, so water drains out properly. Using a good quality peat-free compost is also important, and you can add sharp sand to improve drainage further. Also make sure that your watering can is kept very clean and that the water you are using for small seedlings and cuttings is from a reliably clean source. Finally also make sure you don't sow too thickly - if seedlings are crowded airflow will be poor, and damping off more likely.

• How to prevent damping off

I have tried to grow melons in my unheated greenhouse for the last couple of years. However, I have been unable to get any fruit from them. How do I achieve fruit this year from new plants I am growing now? Sue, North-west England

Matt Biggs says: ‘Alvaro’, ‘Ogen’ and ‘Emir’ are suitable in Britain, and the latter is best for northern counties. Female flowers have a rounded fruitlet behind the flower, while males don’t. Remove the petals from a male flower and gently push the anthers into a female – one male pollinates around four females. Alternatively, transfer the pollen using a fine artists' paintbrush. When the melon fruits are the size of a gooseberry remove all but the best two from each sideshoot, leaving up to six per plant, then feed weekly with tomato fertiliser until late summer as the fruits ripen. Pinch out the shoot tips two or three leaves past the outermost fruit, and any later new growth or flowers, to encourage the plant to put its energy into fruit production rather than green growth.

How can I revive an old beech hedge? We have removed lots of ivy and all sorts of other plants so it does look better, but now there are big gaps. How would you suggest I fix those? Gillian, East Lothian

Ashley Edwards says: Beech will respond well to a regenerative prune in winter when dormant. I’d recommend doing this in stages to avoid shocking the plant too much. In the first year, prune one side of the hedge hard, almost back to the main trunk if necessary, but trim the other side and the top just as you would normally. In the second year prune the other side in the same fashion, giving the top and the other side just a standard trim. In the third year you can reduce the height dramatically, just trimming the sides as you normally would, making sure that the hedge is thicker at the bottom than at the top, so light can reach all of the foliage better. You need to leave a whole season between these prunings to allow the plant to recover. Feed with a good organic compost mulch annually, as it will be balanced and add nutrients over a long period of time.

Our experts

Ashley Edwards

As a head gardener with charity Horatio’s Garden, Ashley enjoys sharing his passion for plants

Humaira Ikram

Humaira has worked as a garden designer for over 10 years and runs a garden design course

Christine Walkden

Garden writer Christine appears on BBC1's The One Show. She is also a lecturer and tour leader

Matt Biggs

Matt trained at Kew and has been gardening professionally for more than 30 years

More of your questions answered:

• Your March questions answered
• Your February questions answered
• Watch Adam Frost's answers to your questions in January